Diet of the Nankeen Kestrel Falco Cenchroides on Christmas Island
28 AUSTRALIAN Field Ornithology 2009, 26, 28–32 Diet of the Nankeen Kestrel Falco cenchroides on Christmas Island
MARTIN SCHULZ1 and LINDY LUMSDEN Arthur Rylah Institute for Environmental Research, Department of Sustainability and Environment, 123 Brown Street, Heidelberg, Victoria 3084 1Present address: Scientific Services Division, Department of Environment and Climate Change, P.O. Box 1967, Hurstville, New South Wales 2220 (Email: [email protected])
Summary. This note reports on the diet of the Nankeen Kestrel Falco cenchroides on Christmas Island (Indian Ocean) from remains located at perches in 2005 and 2008. The dominant prey item was the Giant Grasshopper Valanga irregularis. Other prey included the Christmas Island Swiftlet Collocalia linchi natalis and the introduced Grass Skink Lygosoma bowringii.
The Nankeen Kestrel Falco cenchroides is a partially migratory and dispersive species (Olsen & Olsen 1987) that has expanded its range in recent decades, colonising far-offshore islands, including Lord Howe Island in the 1940s and Norfolk Island in the 1960s (Schodde et al. 1983; McAllan et al. 2004). On Christmas Island (Indian Ocean) the species first arrived in the 1940s and was initially present in low numbers in the north-eastern section of the island (Rumpff 1992). By the 1980s the species had expanded its range and significantly increased in abundance (H. Rumpff cited in Lumsden et al. 1999) and still appears to be increasing, although this has not been quantified (Lumsden et al. 2007). Until this species’ colonisation, the only resident raptor on the island was the Variable Goshawk Accipiter hiogaster. The successful establishment of the Kestrel has been associated with habitat alteration resulting from phosphate-mining operations, causing the creation of extensive open minefields. The Kestrel is now widespread across the island, including occurring over primary rainforest adjacent to open habitats. It is commonly observed feeding in regenerating rainforest areas and the extensive minefields, which are characterised by limestone pinnacles and thin soil supporting either bare ground or low vegetation typically dominated by exotic plant species. The impact of this colonising raptor, as a result of human-induced habitat changes, on endemic island fauna has not been documented. The current study investigated the diet of the Kestrel during two visits to the island, in December 2005 and May 2008. In mined areas, the Kestrel frequently perches on the crests of limestone pinnacles or bare dirt mounds, from which individuals scan for potential prey or fly back to consume prey after capture (Plate 8). Accessible pinnacles were searched for prey remains left by the Kestrel in extensive minefields: Fields 25 and 26 in December 2005 and Field 25, on South Point and along the East-West Baseline, in May 2008. Unlike other studies elsewhere (e.g. Gennelly 1978; Debus 1981; Dickman et al. 1991), no pellets were located; instead, all perches were signified by a variety of discarded prey items amongst varying amounts of whitewash. Collection of remains at perches may be advantageous, in that several previous authors recorded the stripping of wings and legs of larger insect prey and the skinning of mammalian prey at feeding stations before consumption (Hayward & MacFarlane 1971; Genelly 1978). Because of the frequent heavy rain on the island, most of VOL. 26 (1–2) march–june 2009 diet of Nankeen Kestrel on Christmas Island 29
Remains of the Giant Grasshopper Valanga irregularis at a feeding perch on the crest of a limestone pinnacle in a minefield, Christmas Island Plate 8 Photo: Martin Schulz the prey items were collected in small accumulations in recesses or around other obstructions rather than on bare flat surfaces. Where discarded prey remains were found, the number of individuals of each prey species was estimated based on the number of leg pairs (e.g. Orthoptera) or the number of elytra/wings (Coleoptera and Lepidoptera). The presence of other prey items was identified on the basis of recognisable fragments, and was scored as one item per perch. Ninety-six perches with prey remains were located in December 2005 and 19 in May 2008, on pinnacles or dirt mounds respectively in extensive mined areas. Additionally, prey remains were also found under two power-pole perches along Phosphate Hill Road and under three road signs on Vagabond and Lily Beach Roads during December 2005. In total, 2338 individual prey items were identified when both survey periods were combined, of which 97% comprised the Giant Grasshopper Valanga irregularis (Table 1). This species was recorded at 100% of the feeding sites. Smaller numbers of other insects (Orthoptera, Coleoptera, Lepidoptera, Blattodea and Mantodea) were recorded. The only vertebrates identified were the introduced Grass Skink Lygosoma bowringii (remains found at three perches) and the Christmas Island Swiftlet Collocalia linchi natalis (feathers, wing-bones and other skeletal remains found at 10 perches). No small mammals, such as the Christmas Island Pipistrelle Pipistrellus murrayi, were recorded as prey items. The prey recorded taken by the Nankeen Kestrel in this study falls within the dietary range taken by the species on the Australian mainland (Marchant & Higgins 1993; Aumann 2001; Debus et al. 2007). Of interest was the taking of the Christmas Island Swiftlet, an endemic subspecies. Predation of the Swiftlet might AUSTRALIAN 30 SCHULZ & LUMSDEN Field Ornithology
Table 1 Prey remains of the Nankeen Kestrel collected from feeding perches on Christmas Island (all areas and both sampling periods, December 2005 and May 2008, combined). Prey species Total % total prey No. of feeding % of perch number items perches sites
Invertebrates: Giant Grasshopper 2267 97.0 120 100.0 Valanga irregularis Other grasshoppers (Orthoptera) 3 0.1 2 1.7 Christmas Island Jewel Beetle 33 1.4 21 17.5 Chrysoderma simplex Click beetle (Elateridae) 8 0.3 1 0.8 Unidentified beetles (Coleoptera) 3 0.1 3 2.5 Meadow Argus Junonia villida 8 0.3 6 5.0 (Lepidoptera) Unidentified cockroach (Blattodea) 2 0.1 2 1.7 Unidentified mantis (Mantodea) 1 <0.1 1 0.8 Reptiles: Grass Skink Lygosoma bowringii 3 0.1 3 2.5 Birds: Christmas Island Swiftlet 10 0.4 10 8.3 Collocalia linchi natalis Total 2338 100 120 be expected, given that on the mainland other small aerial-feeding birds, such as the Welcome Swallow Hirundo neoxena and the White-breasted Woodswallow Artamus leucorynchus, have been recorded taken occasionally by the Kestrel (records cited by Marchant & Higgins 1993). No direct observations were made of Kestrels taking the Christmas Island Swiftlet, but Swiftlets may have been captured either on the wing whilst foraging or as they emerged from roosting caves on the edge of primary rainforest. The impact of the self-introduced Nankeen Kestrel on the endemic island fauna is poorly known. It is likely that the greatest impacts occurred in the 1980s when the species’ population rapidly expanded. At this time little attention was focused on the Kestrel, compared with a human-introduced predator, the Asian Wolf Snake Lycodon capucinus that arrived in the mid 1980s (Smith 1988; Rumpff 1992). However, around this time the Kestrel was partially implicated in the decline of the endemic Blue-tailed Skink Cryptoblepharus egeriae around The Settlement (Rumpff 1992). More recently it has been suggested to be a factor in the decline of the Foreshore Skink Emoia atrocostata (Schulz & Barker 2008). Both these lizards commonly bask in open situations, making them susceptible to predation by this raptor, similar to a variety of other small diurnal skinks on the Australian mainland (e.g. Olsen et al. 1979; Dickman et al. 1991; Paull 1991; Starr et al. 2004). It has also been suggested that the Kestrel may have contributed to the decline of the Christmas Island Pipistrelle (Schulz & Lumsden 2004). Although no predation on this bat by the Kestrel has been observed (e.g. Lumsden et al. 1999), there appears to have been a shift in this bat’s foraging behaviour, with bats VOL. 26 (1–2) march–june 2009 diet of Nankeen Kestrel on Christmas Island 31 restricted to foraging at night. In contrast, in the early 1980s when the Kestrel was less abundant, Tidemann (1985) observed the Pipistrelle to be active in the late afternoon. Foraging by bats during daylight hours on islands elsewhere in the world has been correlated with a lack of avian predators (Speakman 1995). It could be speculated, although it cannot be confirmed, that the Christmas Island Pipistrelle has altered its activity patterns, delaying emergence until dusk when the risk of predation is lower, in response to the new predation pressure of the self-introduced Nankeen Kestrel, a known predator of small bats (Marchant & Higgins 1993). The impact of this raptor on the Christmas Island Swiftlet is unknown. However, results from this study indicate that its impact needs to be investigated further, and it is recommended that the overall population be monitored. Similarly, the impact of the Nankeen Kestrel through dietary overlap with the Variable Goshawk has not been investigated but requires assessment.
Thanks to Clive Barker for assisting in locating perches in the 2008 field trip and Dave James for identifying some of the invertebrate prey items. The 2005 fieldwork was conducted as part of an investigation of the threats to the Christmas Island Pipistrelle, and was funded by Parks Australia North through the Arthur Rylah Institute for Environmental Research. The 2008 fieldwork was conducted as a component of a terrestrial reptile survey of the island funded by Parks Australia North.
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Received 23 December 2008